Standard parts of nuclear reactors are the fuel elements forming the core of the reactor that contains the nuclear fuel. Although the fuel elements may assume any one of a number of geometric cross-sections, the elements are comprised of nuclear fuel enclosed by cladding. The cladding is ideally corrosion resistant, non-reactive and heat conductive. Coolant, typically demineralized water, flows in the flow channels that are formed between the fuel elements to remove heat from the core. One of the purposes of the cladding is to separate the nuclear material of the fuel from the coolant. Another purpose of the cladding is to minimize or prevent the radioactive fission products from contacting the coolant and thereby being spread throughout the primary cooling system. However, over time different cladding designs have failed by a number of failure mechanisms.
In order to accomplish these and other purposes, various materials and combinations of materials have been used in the cladding. The most common cladding materials include zirconium and alloys of zirconium, stainless steel, aluminum and its alloys, niobium and other materials. Of these, zirconium and its alloys have proven to be excellent materials for such purposes in water reactors because of material properties suited for cladding, including good heat conductivity, good strength and ductility, low neutron absorptivity and good resistance to corrosion.
One composite system utilizes an inner lining of stainless steel metallurgically bonded to zirconium alloy. The disadvantage of this system is that the stainless steel develops brittle phases that ultimately crack, allowing the by-products of the fission to contact the zirconium alloy cladding, initiating the deterioration of the zirconium alloy outer cladding. Furthermore, the stainless steel layer has a neutron absorption penalty of ten to fifteen times the penalty for a zirconium alloy of the same thickness. A solution to the problem of cladding failure is set forth in U.S. Pat. No. 3,969,186 which sets forth a composite consisting of refractory metals such as molybdenum, tungsten, rhenium, niobium and alloys of these materials in the form of a tube or foil of single or multiple layers or a coating on the internal surface of the cladding.
Still another solution to the problem is set forth in U.S. Pat. No. 4,045,288 that teaches the use of a composite cladding of zirconium alloy substrate with a sponge zirconium liner. The concept is that the commercially pure, soft, ductile zirconium liner minimizes the localized strain that the outer cladding is subject to. However, if a breach in the outer cladding should occur, allowing water and/or steam to enter the fuel rod, the zirconium liner tends to oxidize rapidly.
Yet another approach to the problem of cladding failure set forth in U.S. application Ser. No. 06/374,052 filed May 3, 1982, assigned to the assignee of the present application, and incorporated herein by reference, teaches using a composite cladding consisting of a dilute zirconium alloy inner liner metallurgically bonded to conventional cladding materials such as zirconium alloy claddings. The dilute zirconium alloy inner liner includes at least one metal alloyed with the zirconium selected from the group consisting of iron, chromium, iron plus chromium and copper. The amount of iron alloyed with the zirconium is from about 0.2% to about 0.3% by weight; the amount of chromium is from about 0.05% to about 0.3% by weight; the total amount of iron plus chromium is from about 0.15% to about 0.3% by weight and wherein the ratio of the weights of iron to chromium is in the range of from about 1:1 to about 4:1; and wherein the amount of copper is from about 0.02% to about 0.2% by weight.
While advances have been made in the area of improving the performance of claddings, corrosion and brittle splitting of the cladding due to interactions of the cladding, the nuclear fuel, the fission products and the coolant continues to be a problem even with the improved systems.